1. Field of the Invention
The present invention relates to an exposure apparatus, and more particularly, a die-by-die, or shot by shot exposure apparatus for manufacturing semiconductor circuits, and provides an apparatus which can reduce time required in the operational step from the alignment to the exposure at each shot.
2. Description of the Prior Art
There is generally known semiconductor manufacturing exposure systems in which the pattern on a mask or reticle (hereinafter called generally a "mask") as an original is printed on a wafer. Recently, more attention has been attracted to an die-by-die type exposure system (a so-called stepper).
In such a stepper, the pattern on a mask is projected and printed on a wafer at a reduced scale through a projection optical system having its magnification less than unit. To avoid any overlapping in the exposed areas at each shot, the mask and wafer are moved relative to each other so that the pattern will be printed on all the effective areas of the wafer through a plurality of shots.
With the die-by-die exposure utilizing the above projection at the reduced scale, the pattern on the mask can be made larger than the pattern to be printed on the wafer by a factor of the inverse number of the magnification in an imaging system. Thus, difficulties in manufacturing marks can be eased so that finer patterns can be printed on wafers.
The stepper is generally classified into two types, that is, (1) the OFF-AXIS alignment type and (2) the TTL (Through The Lens) ON-AXIS alignment type.
In the off-axis alignment system, the pattern on a wafer is aligned with an alignment optical system located out of a projection optical system without passing through the latter. Thereafter, the wafer is accurately moved below the projection optical system such that the wafer pattern will be aligned indirectly with the pattern on a photo-mask.
In the TTL on-axis alignment system, the patterns on a photo-mask and wafer are simultaneously observed to align them directly with each other. The stepper should have a precision of alignment on the order of 0.1 to 0.2 .mu.m. The off-axis alignment system has a larger number of factors of error, because of the fact that the wafer pattern is aligned indirectly with the photo-mask pattern and also cannot deal with inplane-distortion in the wafer. To improve the precision of alignment, therefore, it is highly desirable to develop the TTL on-axis alignment system.
The prior art TTL on-axis alignment type steppers utilize the following alignment process:
(1) Displacement, i.e., the misalignment between a mask and wafer is measured through a binocular microscope which is positioned in an optical path of illumination. For example, as disclosed in U.S. Pat. No. 4,167,677, alignment marks of a particular shape on the mask and wafer are scanned by a laser beam such that the degree of misalignment can automatically be measured in an photoelectric manner.
(2) The degree of misalignment measured in the above step (1) is corrected by moving a stage on which the mask or wafer is placed.
(3) Whether or not the correction is properly made is verified by the use of the binocular microscope which is located in the optical path of illumination.
(4) The binocular microscope is retracted out of the optical path.
However, the TTL on-axis alignment system has a problem in that longer time is required in the operation from the beginning of the alignment to the exposure at each shot.
Further, during the entire exposure step, two alignment marks to be observed through the binocular microscope at each shot should be maintained at such a state that the alignment marks can be used. In addition, if the alignment mark on the outer margin of the exposed region on the wafer is partially broken, no alignment operation can be made.